Use map to insert and delete data can case memory leak

[FengtuWang]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.9.2 linux/amd64

Does this issue reproduce with the latest release?

Yes, latest release download from https://golang.org/dl/

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build363981589=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Use map to insert and delete data, code like this:

package main

import (
	"fmt"
	"runtime"
	"runtime/debug"
)

func main() {
	a := make(map[int]int)

	stats := new(runtime.MemStats)
	runtime.ReadMemStats(stats)
	fmt.Printf("Before: %5.2fk\n", float64(stats.Alloc)/1024)
	previous := stats.Alloc

	for i := 1000000; i > 0; i-- {
		a[i] = 0
	}

	for k := range a {
		delete(a, k)
	}

	debug.FreeOSMemory()
	fmt.Printf("len(a)=%v\n", len(a))

	runtime.ReadMemStats(stats)
	fmt.Printf("After: %5.2fk    Added: %5.2fk\n", float64(stats.Alloc)/1024, float64(stats.Alloc-previous)/1024)

	return
}

What did you expect to see?

Memory not increased obviously

What did you see instead?

Memory increased obviously, output like this:

Before: 42.54k
len(a)=0
After: 39252.77k    Added: 39210.23k

I do not know why so many memory still exist after i delete all elements in map.

[mvdan]

Dup of #20135.